Flutter: how do I solve this error when trying to query for a map within an array in Firestore

Question

In this project, I have image details stored in a map, within an array (imageUrls) in Firestore. I'm trying to query for the map that contains a specific imageName, then delete that map. My console isn't throwing any errors, and I've confirmed my Firestore rules are read & write.

Here's what my database looks like.

(https://i.stack.imgur.com/2UZpL.png)

And my corresponding code.

@override
  Future<void> deleteUserPictures(User user, String imageName) async {
    try {
      var userImagesCollection = await _firebaseFirestore
          .collection('users')
          .doc(user.id)
          .collection('imageUrls')
          .where('imageName', isEqualTo: imageName)
          .get();

      // Loop through the documents that match the query
      for (var docSnapshot in userImagesCollection.docs) {
        // Delete the document
        await docSnapshot.reference.delete();
      }
    } catch (e) {
      print('Error deleting image: $e');
      throw e;
    }
  }

Thanks in advance for any help!